The present invention relates to an apparatus and method for filling a target site in a living being with an embolic material to occlude the site; and, more particularly, to an apparatus and method for automatically notifying an operator of the instant the embolic material is detached from a guiding member by electrolytic action.
There is an operative treatment of treating vascular malformations such as a cerebral aneurysm, which includes the processes of putting a patient under general anesthesia to craniotomy, exposing the cerebral aneurysm in the patient using an operating microscope and a microsurgical unit, and clipping a cervical portion of the cerebral aneurysm with a particular metallic clip. However, such a treatment suffers from drawbacks that it still involves a considerable hazard and a prolonged operating time, which, in turn, may cause serious sequelae.
In an alternative treatment, a Minimal Invasive Treatment (MIT), which employs a technique disclosed in U.S. Pat. No.5,122,136 issued to Guglielmi et al, and U.S. Pat. Nos.4,884,579 and U.S. Pat. No. 4,739,768 issued to Engelson, is utilizing. The MIT Treatment inserts an embolic material within vascular malformations such as a cerebral aneurysm through the use of a micro catheter and a guiding wire under fluoroscopy to occlude the vascular malformations. In contrast with the craniotomy treatment previously explained, the MIT treatment has merits that it is possible to operate under a slight anesthesia with a short operation time, to thereby minimize sequelae and also lower an operation cost.
An embolic material mainly utilized in the MIT treatment includes a metallic coil. The metallic coil is disclosed in, for example, U.S. Pat. Nos. 5,354,295, 5,669,905 and 6,066,133 and Japanese Patent Nos. 10-057385, 11-047138 and 11-076249. In the following, the metallic coil cited in U.S. Pat. No. 5,669,905 will be described.
FIG. 1 is a pictorial view of a metallic embolic coil used in the conventional MIT treatment.
As shown in FIG. 1, a guiding wire assembly 100 typically includes a stainless steel-based guiding wire 1 and a coil-shaped embolic material 8, the guiding wire 1 being tapered its distal end and the embolic material 8 being connected with the distal end of the guiding wire 1 by a micro welding. The embolic material 8 is made of a radiopaque material including Platinum, Tungsten, Iridium or these alloys, and has welded portions 6 and 7 at its both ends. The welded portions 6 and 7 are made of platinum that acts as a marker under fluoroscopy.
A surface of the guiding wire 1 is coated with an insulating material such as Teflon, with the exception of a proximal end 5 acting as a sacrificial link to be connected with the welded portion 6 of the embolic material 8. The sacrificial link 5 is made of an electrically conducting material such as stainless steel, which is a portion to be detached from the guiding wire 1 by electrolytic disintegration. The guiding wire 1 is coupled with the welded portion 6 of the embolic material 8 via the sacrificial 5, which is interposed in a sleeve 2 and a plug 3 with inserted within an internal coil 4. The internal coil 4 is designed to provide column strength to the guiding wire 1, without a bad influence for a flexibility of the tapered portion in the guiding wire 1. As shown in FIG. 1, the embolic material 8 has been designed its shape changed into a coil form when it is gradually withdrawn from a micro catheter 7, to thereby allow the embolic material to adapt to the shape of the vascular malformation.
FIG. 2 is a pictorial view illustrating insertion and detachment processes of the embolic material 8 in the prior art.
Typically, the insertion of the embolic material 8 in a vascular malformation 11 is performed using fluoroscope under local anesthesia. Specifically, as shown in FIG. 2A, an operator guides a micro catheter 10 to near neck 12 of the vascular malformation 11 in a living being or a patient. After that, the operator inserts the guiding wire 1 attached the embolic material 8 on its distal end into the micro catheter 10, and gently push the guiding wire 1 using the fluoroscope at least until the sacrificial link 5 is exposed beyond the distal end of the micro catheter 10.
In an ensuing step, an electrical loop is formed wherein a positive electrode of a power supply 13 is attached to the proximal end of the guiding wire 1 and a negative electrode is placed in electrical contact with the skin of the patient. Thereafter, the power supply 13 is turned on to allow a DC power with AC superposition to be applied to the embolic material 8 through the sacrificial link 5 of the guiding wire 1. As a result of the above process, the embolic material 8 is detached from the guiding wire 1 by electrolysis as shown in FIG. 2B. Next, the guiding wire 1 and the micro catheter 10 are withdrawn from the vascular malformation 11.
FIG. 3 shows a schematic block diagram of the prior art apparatus of detecting the detachment of the embolic material from the guiding wire.
The prior art apparatus 200 includes a constant current source 16, a circuit 18 for detecting the detachment of the embolic material and a microprocessor 19. The constant current source 16 provides a constant current to the patient 17, which includes an operational amplifier (OP Amp) 16a and a DC feedback loop 16b. The OP Amp 16a will oscillate in approximately 30 kHz at amplitude of several hundred milli-volts due to a lagging error correction signal (out-of-phase feedback). That is, the OP Amp 16a provides a DC current with AC superposition. The amplitude of such AC signal is dependent on bandwidth characteristics of the OP Amp 16a, AC impedance of the stainless steel and the embolic material 8, and the patient""s body. The DC constant current flowing out of the OP Amp 16a flows through the sacrificial link 5 of the guiding wire 1 to the embolic material 8.
Although the sacrificial link 5 and the embolic material 8 are physically connected in series, immersion of them in an electrolytic solution forms two parallel DC current paths each of that is grounded through the body of the patient 17. Specifically, by ion flow away from the stainless steel-based link 5 during electrolysis, the DC current with AC superposition flowing between the sacrificial link 5 and the embolic material 8 in the vascular malformations 11 is branched as follows. That is, the majority of the DC current (above 99%) flows through the sacrificial link 5 with the remaining (less 1%) flowing through the embolic material 8. Thus, if the embolic material 8 is separated from the link 5 and a portion of the sacrificial link 5 remains attached to the guiding wire 1, the main DC current is fed back to the DC feedback loop 16b of the constant current source 16. The AC current is grounded through the embolic material 8.
As shown in FIG. 3, the DC current with AC superposition is blocked out by a pick-off capacitor (not shown), only the AC signal is fed to the detection circuit 18 for measurement of AC impedance. The detection circuit 18 receives the AC current from the embolic material 8 in the patient 17 to detect whether or not the embolic material 8 is detached. Specifically, the AC current fed to the detection circuit 18 is amplified in an AC signal amplifier 18a and is rectified in an AC-DC rectifier 18b. Then, the rectified DC signal is amplified in a DC level amplifier 18c and sent to the microprocessor 19, wherein the amplified DC level is representative of the amplitude of the AC voltage of the OP Amp 16a. 
The microprocessor 19 monitors the level of the amplified DC signal every 50 to 200 milliseconds and constantly averages the signal every specific sample. In this manner, if a sudden DC voltage drop is incurred, the microprocessor 19 determines that the embolic material 8 is detached from the guiding wire 1.
In the prior art, the OP Amp 16a oscillated on its own, which allowed the monitoring of the AC impedance fluctuation by the detection circuit 18. Since, however, there were fluctuations in the self-oscillation signal flowing- from unit to unit, it fails to exactly determine the instant the embolic material is detached. That is to say, a fluctuation in the AC impedance depends on a length of the embolic material and other physical factors, thereby invoking poor detachment detection.
To support this, as shown in FIG. 4, an external AC source 20b is utilized to ensure all units will show the identical response to the fluctuation in the AC impedance. In FIG. 4, the AC source 20b is coupled with a reference input Vref of an OP Amp 20a so as to modulate the output current of the OP Amp 20a (i.e., provide AC superposition on the DC current). A DC current with AC superposition is outputted from the OP Amp 20a and sent to the embolic material 8 through the sacrificial link 5 of the guiding wire 1. As a result, two AC and DC current paths branch as described above with reference to FIG. 3. The DC current with AC superposition from the patient 17 is fed back to an AC and DC feedback loop 20c of a constant current source 20 and fed to the OP Amp 20a. 
As stated above, the DC current with AC superposition is blocked out by a pick-off capacitor (not shown), only the AC signal is fed to the detection circuit 21 for measurement of AC impedance fluctuation. In the detection circuit 21, since the amplitude of the AC signal is substantially greater than that of FIG. 3, the DC level amplifier 18c in FIG. 3 is not necessary. As noted, the AC signal is amplified in an AC signal amplifier 21a in the detection circuit 21 and is rectified in an AC-DC rectifier 21b. Then, the rectified DC signal is sent to the microprocessor 19.
In short, the prior art apparatuses previously disclosed detect the detachment of the embolic material using the AC signal. Accordingly, the prior art apparatuses suffer from a drawback that if a fluctuation in the AC impedance depends on a length of the embolic material and other physical factors, it is difficult to exactly detect the detachment instant. In addition, the prior art guiding wire assembly for embolization demands to an additional coil for maintaining the shape of the guiding wire and an additional signal source, thereby rendering the apparatus rather complex and costly. Likewise, although the embolic material in the prior art has been fabricated in platinum, tungsten, gold, iridium or alloy for thrombus in vascular malformations, it would be desirable to effectively enhance the rate of thrombus without any application of high power to the material.
It is, therefore, an object of the present invention to provide a system and method for exactly determining the instant an embolic material is detached through the use of a single power supply, without any additional circuits.
It is another object of the present invention to provide an assembly for embolization, which includes a simplified guiding member and an embolic material inducing an improved thrombus in vascular malformations.
In accordance with a preferred embodiment of the present invention, there is provided to a system for detecting the detachment of an implant from a guiding member coupled thereto, wherein the implant is guided by the guiding member into a target site in a living being, comprising: means for generating a current; means for supplying the generated current via the guiding member to the implant; means for measuring a voltage and current between the guiding member and the ground for a plurality of cycles, during each of which the voltage and current are measured for a plurality of times; means for computing the average of the measured voltages and the average of measured currents in each cycle; and means for determining the detachment of an implant from the guiding member based on a change in impedance between the guiding member and the ground, wherein the change is detected using the average voltage and average current of the latest cycle of voltage and current measurements, and the average voltage of the previous cycle of voltage and current measurements.